1. Field of the Invention
The present invention relates to a process of injection stretch blow molding a hollow article having a thick-walled bottom, such as a refillable and returnable bottle (hereinafter called "R-R bottle") or pressure vessel subjected to an internal pressure from a carbonated beverage contained therein.
2. Description of the Related Art
R-R bottles have been developed to meet requirements in the reuse of resources and can be repeatedly used. The used R-R bottles are washed with caustic soda at a raised temperature and subjected to various tests such as a heat stability test at about 38 degrees C. or the like. If the bottom walls of the R-R bottles are too thin, they cannot withstand a predetermined number of repeated uses due to cracking or crazing. On the other hand, a pressure vessel subjected to the internal pressure generally has its inwardly concave bottom wall which is known as a champagne bottle's bottom. If the bottom wall of such a pressure vessel is too thin, the bottom wall may be undesirably deformed to be outwardly concave under the internal pressure of the pressure vessel. It is thus required in the art that the bottom walls of such bottles and/or vessels are larger in wall-thickness than those of the barrel portions.
In general, such a bottle having its thick-walled bottom may be molded by controlling the wall thickness of a preform and also the temperature condition in the preform. In the hot parison system wherein a preform (parison) maintaining heat applied thereto on the injection molding step is biaxially stretch blow molded, however, the wall-thickness and temperature condition must be experientally determined. This renders the design of the pressure bottle and/or vessel very difficult since the champagne bottle's bottom may be too thin or since the bottle bottom may have crater-shaped thick wall portions when the preform is caused to have an excessive wall thickness.
The provision of thick-walled bottom to a vessel or bottle has been proposed, for example, in Japanese Patent Laid-Open No. Sho 64-5815 or Japanese Patent Laid-Open No. Hei 2-128826.
In these prior techniques, a preform has a barrel portion and a bottom wall, the barrel portion having a thick-walled part at a position near the bottom wall of the preform. The thick-walled part of the barrel portion is formed, at its inner and top wall, with a step, a taper or a projection which is adapted to engage the distal end of a stretching rod mechanically. When the stretching rod engages the step, the taper or the projection of the barrel portion, said thick-walled part of the barrel portion near the bottom wall of the preform is less subjected to the longitudinal stretching force from the stretching rod. Thus, the longitudinal stretching operation can be performed while maintaining the longitudinal stretching force acting on the upper region of the preform larger (see FIGS. 9, 17 and 18 of Japanese Patent Laid-Open No. Sho 64-5815 and FIG. 7 of Japanese Patent Laid-Open No. Hei 2-128826). Thus, the thick-walled part of the barrel portion adjacent the bottom wall of the preform can be maintained at a wall thickness substantially equal to that provided on the injection molding step substantially immediately before the end of the longitudinal stretching step while the remaining parts are being stretched. A blow air (secondary blow air in Japanese Patent Laid-Open No. Hei 2-128826) is thereafter introduced into the preform to stretch the thick-walled barrel part into engagement with the inner wall of the blow mold. The thick-walled barrel part adjacent the bottom wall can be formed into a wall thickness larger than that of the upper barrel part.
One of the techniques relating to the present invention is disclosed in Japanese Patent Publication No. Sho 50-13829 which describes a technique of regulating the temperature of a preform to be stretch blow molded, although it does not aim to mold the thick-walled bottom of a bottle. This technique regulates the temperature of a preform bottom such that it will not be penetrated by the stretching rod while the latter is in contact with the inner bottom wall of the preform, in a re-heating step before the injection molded preform is moved to the blow molding stage. Such a technique is an early developed temperature regulating system for hot parison, in which the entire preform is regulated uniformly to a temperature suitable in stretching to secure a desirable distribution of temperature along the longitudinal axis of the preform (i.e. lower temperature at the bottom wall) such that the bottom of the preform will not be penetrated by the stretching rod.
In the molding techniques disclosed in the aforementioned Japanese Patent Laid-Open No. Sho 64-5815 and Japanese Patent Laid-Open No. Hei 2-128826, a preform having a specific configuration in which a thick-walled part is formed in the barrel portion of the preform at a position adjacent to the bottom wall of the preform and in which the preform has a rapid transition in wall thickness between said thick-walled part and the remaining parts of the barrel portion must be injection molded. This renders the workability of the injection mold and particularly the core rod difficult. Furthermore, it is very difficult not only to set various conditions on injection molding such a preform, but also to regulate the temperature of the preform before blow molding. It is particularly rare that a desired wall thickness in the bottom of a bottle is obtained as the design. The control of wall thickness must be attained by re-setting the injection conditions in accordance with changes in the core rod. Such an operation is extremely cumbersome. On the other hand, the process of biaxially stretch blow molding a preform into a bottle has an inherent temperature suitable in stretching, at which the portion below the neck portion has generally a temperature higher than that of the bottom portion. Where the barrel wall portion of the preform at a position adjacent to the bottom wall portion is thick-walled, the thick-walled barrel part has its increased heat capacity which will maintain a level of temperature given by the injection molding step higher. It is thus very difficult to regulate the temperature of the thick-walled bottom part to a reduced level. In order to regulate the temperature of the aforementioned preform having its specific configuration, it must be secured that the bottom wall part having its increased heat capacity is regulated to a lower temperature while the thin-walled barrel part having its decreased heat capacity is regulated to a higher temperature.
The aforementioned Japanese Patent Laid-Open No. Sho 64-5815 and Japanese Patent Laid-Open No. Hei 2-128826 do not disclose the control of temperature on a specific preform at all. In order to realize the above technique, however, it is essential to accomplish a difficult goal that the specific preform can be regulated into an appropriate temperature for stretching.
The aforementioned Japanese Patent Publication No. Sho 50-13829 discloses a technique of regulating the temperature of the bottom preform wall by the use of a stretching rod at the reheating step different from the blow molding step. However, this technique does not teach and suggest that the bottom wall of a bottle should be maintained at a desirable wall thickness, but only describes the use of a stretching rod contacted by the inner bottom wall of the preform to regulate the temperature thereof. This is probably sufficient to prevent any penetration of the bottom preform wall by the stretching rod. It is, however, apparent that the wall thickness of the final product will be adversely affected by changes in temperature between the reheating step and the blow molding step and yet that the desired wall thickness at the bottom wall required in R-R bottles and the like cannot be secured only by regulating the temperature of the inner bottom wall of the preform.